If you Google “vb mdi sdi” you will find many resources with examples of how to work with multiple forms.
I am guessing your default forms interface in your development environment is SDI (or Single Document Interface). In this mode, the application opens one main form window, and all other form windows open within the first one. Each form within the main form has the main form as a “parent”, so you cannot close the parent without closing the “child” windows.
MDI is Multiple Document Interface. Generally in this approach, you start a “Main” procedure which then launches one or more forms as needed. The “child” forms are then children of the application, not children of the first form window opened. Thus, your main procedure can open a startup window, collect some info (like userid/password) and then that window can be closed and other windows opened as needed.